The present invention relates to nano-materials, and more particularly to a new class of stabilized, chemically reactive, metallic nano-materials effective for degradation of chlorinated organic compounds in soils, sediments, and groundwater.
Polychlorinated biphenyls (PCBs), tetrachloroethylene (PCE), and trichloroethylene (TCE) are categorized as chlorinated hydrocarbons, which have been widely used in industries and caused serious groundwater and subsurface contamination in hundreds of sites in the U.S. All three chemicals, as well as their intermediate by-products vinyl chloride (VC), cis-dichloroethene (c-DCE) and trans-dichloroethene (t-DCE), are believed to be potent carcinogens.
PCBs were used in hundreds of industrial applications (e.g., in electrical transformers and as plasticizers in paints and plastics) for their non-flammability, stability, and electrical insulating properties. More than 1.5 billion pounds of PCBs were manufactured in the United States from its first industrial use in 1927 to the cessation of production in 1977. The U.S. EPA estimates that about half of the total domestically consumed PCBs (625,000 tons) were dumped into the environment (soils, sediments, and groundwater) before the enactment of federal regulations in 1976.
PCE and TCE are organic solvents widely used in dry cleaning and metal rinsing. In the past, large amounts of used PCE and TCE were simply dumped into the ground. As a result, high concentrations of PCE and TCE are commonly detected in areas adjacent to dry cleaners, automobile manufacturers or repair shops, asphalt processing plants, and military bases.
For over three decades, it has been a highly challenging task to remediate soils and groundwater contaminated with chlorinated hydrocarbons. Among the numerous remediation technologies are bioremediation including bio-augmentation, pump-and-treat, thermal treatment, permeable reactive barriers (PRB), and excavation followed by landfill. While the pros and cons of these technologies have been documented, there remains a strong need for developing more effective technologies to destroy chlorinated hydrocarbons. For example, bio-augmentation has been used to enhance the slow biodegradation rate of TCE, PCE and/or PCBs. However, this method is challenged by the fact that there has been lacking an effective method for controlling the delivery and distribution of electron-donors and nutrients in the contaminated zone. Traditional pump-and-treat methods can require decades of treatment time and operation costs. Thermal treatment (e.g. steam injection and radio-frequency-heating) demands a prohibitive operating cost and may cause contaminant re-mobilization. Excavation and subsequent landfill of contaminated soil is costly and environmentally disruptive and is highly restricted in residential or industrial areas.
In recent years, abiotic dechlorination using zero-valent iron, Fe(0), particles has enticed increasing interest. By 2003, commercial granular iron particles had been employed in about seventy PRBs to degrade chlorinated hydrocarbons. However, due to limited reactivity, the dechlorination rate using these iron particles is often too slow to be practically viable. For example, the half life of TCE reduction was found in days or longer. As a result, even more toxic intermediate by-products such as VC were often detected.
Two major strategies have been explored to modify granular iron particles for improving the dechlorination reaction kinetics. The first one is to lower the particle size, which in turn increases the particle surface area. Because the dechlorination reaction is a surface-mediated process, increasing the surface area results in enhanced reaction kinetics. The second modification involves coating iron particles with a small quantity (<1% of Fe) of a catalytic metal such as palladium (Pd). The resultant bi-metallic particles were found much more reactive than the mono-metallic iron particles and may prevent the formation of the toxic intermediate by-products. It was reported that Pd-coated nanoscale iron particles can dechlorinate TCE 10-100 times faster than mm-scale granular iron particles.
Compared to traditional passive processes such as the “funnel and gate” or PRB processes, in-situ injection of nanoparticles holds a number of advantages. For example, it can attack the source zone proactively, and it may offer much faster reaction kinetics. However, to be viable, the nanoparticles are required to offer several critical attributes, including 1) the particles must be dispersible in soils for desired reaction period, 2) they must be able to offer prolonged reactivity, and 3) they must be environmentally safe. Currently, Fe(0)-based nanoparticles for remediation purposes are typically prepared by reducing Fe(II) or Fe(III) in the aqueous phase with a strong reducing agent (e.g., sodium borohydride, NaBH4). Compared to other preparation methods such as micro-emulsion-based methods, sonication assisted methods and sol-gel methods, the water-based approach appears to be more suitable for environmental applications because of the minimal use of environmentally intensive solvents or chemicals. However, due to the extremely large area-to-volume ratio and the extremely high energy and reactivity, the initially formed nanoparticles tend to react rapidly with the surrounding media (e.g. dissolved oxygen (DO) or water) and interact with other particles to form much larger (micron to millimeter) agglomerates in a few minutes, thereby losing their mobility in soils and reactivity rapidly. Because of agglomeration the steady-state mean particle size of the “nanoparticles” is actually about 17.7 μm. Because particles of 3 μm or larger are easily retained by soil matrix, the agglomerated iron particles are highly restricted from reaching the contaminants when injected into the ground.
In an attempt to “stabilize” the iron nanoparticles, i.e. to prevent the resultant nanoparticles from agglomeration and to prolong their reactivity, Mallouk and co-workers employed carbon nanoparticles and poly(acrylic acid) (PAA) as supports or “vehicles” for stabilizing and/or delivering iron-based nanoparticles. These supports serve as dispersants and prevent iron particles from agglomeration by shielding the dipole-dipole interactions, and thereby prolong the reactivity of the particles. Significant enhancement of permeability of iron particles was observed when the supported particles were used in both sands and soils (Schrick B.; Hydtusky, B. W.; Blough, J. L.; Mallouk, T. E. Delivery Vehicles for Zerovalent Metal Nanoparticles in Soil and Groundwater, Chem. Mater. 2004, 16, 2187-2193). However, there remains a need for developing an improved stabilized nanoparticle that is able to provide an in-situ and cost-effective process for destruction of chlorinated hydrocarbons in soil and groundwater.